Coal Preparation » Process Control
The Australian Coal Association Research Program commissioned JKTech to conduct a review of common instruments in Australian coal preparation plants. The objective was to provide information to assist ACARP in directing R&D funds to areas most likely to significantly improve coal preparation performance.
The study conducted during July and August 1996 acquired a considerable body of literature, data and other information from the equipment suppliers listed in Appendix 1. This helped provide a sound basis for carefully managed and lengthy interviews with key personnel from 22 plants, supported by equipment inspections and discussions at eight of those plants.
Data relating to some thousands of individual instrument installations was entered into a database (Appendix 2) for an analysis which yielded the following conclusions. They present few surprises. However the value of the study lies in the confidence with which those conclusions can now be drawn in respect of a broad cross-section of the Australian coal preparation industry.
The following refer to well-engineered installations, but the body of the report shows that many instruments are poorly installed.
Air Flow Sensors
The increasing market penetration of flotation columns is fuelling a growing demand for air flow monitoring. At this stage it seems likely that the demand can be adequately met by existing technologies embodied in orifice, temperature differential and vortex meters.
Bin charge mass monitoring by strain gauges and bin level monitoring by ultrasonics require careful engineering but the technology is industrially mature and reliable.
Most blockage detection requirements can be met by tilt or microwave switches or the more recent vibration switches. Capacitance and resistance probes are also used, and other technologies are available for special circumstances. A problem in many installations is a requirement for regular cleaning.
DP cell technology routinely provides reliable and accurate pressure measurement. Technology also exists for similar performance from local dial gauges, but cheaper options are often selected, leading to poor performance in terms of both availability and accuracy.
A small experience base suggests that reliable units are available from at least three suppliers. An advantage of this technology is that it avoids those accuracy and maintenance problems associated with pneumatics which arise from poor quality of instrument air.
Flotation Pulp Level
For flotation units which operate with a deep froth, it would seem that an approximate measure of the pulp/froth interface level is adequate, and the use of DP cells is appropriate. In conventional flotation, accuracy of measurement would seem to be more critical and the author believes that the identification or development of a reliable, accurate and low-maintenance pulp level monitor would be beneficial.
Flotation Reagent Dosage
Flotation reagent dosage is a field ripe for improvement. Possible avenues include the development of more reliable metering pumps, or the improved automation of Clarkson-type feeders. An alternative is a cascade control system utilising a flow meter with feed-back control to a pump, or to a valve or splitter fed from a head tank or ring main.
Producers who use contractors for instrument maintenance and calibration are generally pleased with the levels of competency and service. Many of those who continue to conduct these functions in-house do so as a result of industrial relations pressures. They point out that, particularly in the 'multi-skilling' environment, it is unreasonable to expect a plant electrician to develop and maintain the specialist skills demanded by the many classes of modern instrumentation.
With low manning levels it can be difficult to schedule frequent cleaning for those instruments that require it - sedimentation monitors, weightometers and various classes of level detectors.
Some respondents emphasised that some sensors are very sensitive to vibration and that care should be taken to avoid vibration / rubbing damage to cables. Although pneumatics were not a core issue for this study, a number of plant personnel emphasised the importance of clean, dry instrument air.
The report also lists 25 potential or developing instruments or measurement-related issues and offers very brief comments in relation to a number of them. Those considered to be both very important and appropriate issues for the attention of ACARP are:
- the control of distribution between parallel circuits and
- the development of widely-applicable strategies for reconciliation of the flows of coal and other materials at all stages from the geological resource through to the end user
The latter issue encompasses units and techniques such as truck dispatch systems, truck weighers, stockpile surveys and draft surveys as well as the weightometer and slurry flow instruments addressed in this report.
Although user perceptions of moisture monitor performances are poor, there is a growing body of evidence that capacitance units, and especially the newer microwave units with nucleonic load compensation, are more accurate than the 'standard' measurements against which they are calibrated. If it can be shown that a large number of situations exist where accurate moisture monitoring is critical, ACARP may consider directing funds to the development of a reliable calibration procedure. It should utilise the Grubbs estimator or similar technique and must be capable of being performed at moderate cost, even for coarse coals.
Position Monitors for Stackers, Reclaimers and Trippers
Systems based on rotary encoders and limit or proximity switches, supported by logic testing are reliable. User satisfaction with such arrangements is generally high.
This relatively new technology for thickener control has proven very valuable. Many units are maintained by the suppliers and thus provide few headaches for operators. The most common problems are relatively minor and do not affect all installations. They relate to vacuum sampling systems and to staining of surfaces through which the light must pass. Further improvement of these units should not be an ACARP priority.
Perrformances of density and percent solids gauges, which use nucleonic or DP cell technologies, are quite adequate.
Doppler and electro-magnetic technologies are adequate for the duties to which they are commonly applied - principally fine coal and tailings streams. A reliable instrument for flow rates of medium streams would be of value - a 'time of flight' monitor utilising pulsed ultrasonics may be suitable. Two manufacturers have developed prototypes which monitor both magnetic properties and flow rate. No details are available.
DP cell and ultrasonic technologies for sump level detection are very satisfactory.
Doppler, electro-magnetic, turbine and vortex meters can meet most requirement of water flow.
Normal calibration procedures are not always adequate, necessitating the incorporation of a bio factor. Top-of-the-line weightometers probably rival the accuracies of weighbridges or other reference techniques. A valid activity for ACARP may be the development of guidelines for improved weigher calibration, giving due consideration to whatever reference measurements are available in relation to specific installations.